The erythrocyte concentrations and the transmembrane fluxes of sodium and potassium were investigated in normal men and also in twenty normal women during the two stages of the menstrual cycle. Half of them were taking the contraceptive pill and the other half were not. In women with a normal menstrual cycle, the erythrocyte sodium concentration (Naic) and the ouabain-insensitive total potassium efflux were lower in the luteal than in the follicular phase. Intracellular potassium concentration (Kic), ouabain-sensitive 86rubidium-uptake and the furosemide-sensitive Na+ and K+ efflux did not differ significantly between the two periods of the cycle. No cycle-related variation in Naic or Kic was observed in women using the contraceptive pill. In these women, however, the ouabain-sensitive 86rubidium-uptake was increased in the second part of the menstrual cycle. Compared to men, the intra-erythrocyte sodium concentration was lower in women during the second stage of the menstrual cycle. These two groups were similar for Na+, K+-ATPase pump activity estimated from the ouabain-sensitive 86rubidium-uptake and for the furosemide-sensitive sodium and potassium efflux. Women in the first stage of the menstrual cycle had intra-erythrocyte sodium concentration similar to men, but their furosemide-sensitive sodium efflux was lower. No significant difference was observed in the intraerythrocyte potassium concentration and transmembrane fluxes of potassium in men and women in either stage of the menstrual cycle. We therefore conclude from this study that one should take into account sex-related variability when studying cationic fluxes and concentrations in red blood cells of men and women.